1. Field of the Invention
The present invention relates to the technical field of liquid crystal displaying, and in particular to a method of defining poly-silicon growth direction.
2. Description of Prior Art
With the development of the flat panel displays, the panels with higher resolution and lower power consumption are constantly required. Unlike amorphous silicon, of which the electron mobility is low, the low temperature poly-silicon can be produced in a low temperature environment. With the high electron mobility and capability of manufacturing the C-MOS (Complementary Metal Oxide Semiconductor) circuit, the low temperature poly-silicon is widely discussed for meeting the requirement of high resolution and low power consumption.
The low temperature poly-silicon (LTPS) is a branch of poly-silicon technology. For the flat panel displays, the application of the poly-silicon liquid crystal material provides lots of merits. For example, the film circuit can be thinner, smaller and having lower power consumption.
In the initial developing stages of the poly-silicon technology, a laser anneal process, which is a high temperature oxidation process is necessary for transferring the structure of the glass substrate from amorphous silicon (a-Si) into poly-silicon. In this moment, the temperature of the glass substrate can reach over 1000 degree C. In comparison with the traditional high temperature poly-silicon, the laser exposure process is still required for the low temperature poly-silicon, thought. Nevertheless, an excimer laser is employed as being the heat source. After the laser is conducted through the transmission system, a laser beam with uniformly distributed energy is projected on the glass substrate with amorphous silicon structure. After the glass substrate with amorphous silicon structure absorbs the energy of the excimer laser, the glass substrate is then transferred into poly-silicon structure substrate. The whole process is accomplished under 500-600 degree C. Even a normal glass substrate can bare such a temperature which enormously reduces the manufacture cost. Beside reduction of the manufacture cost, the low temperature poly-silicon technology further provides more merits: higher electron mobility; smaller film circuit area; higher resolution; simple structure and greater stability.
At present, several methods for producing the low temperature poly-silicon can be illustrated, such as solid phase crystallization (SPC), solid phase crystallization (SPC) and excimer laser anneal (ELA), among which excimer laser anneal (ELA) is the most widely used method nowadays.
The manufacture method of low temperature poly-silicon by excimer laser anneal is to grow a buffer layer on a glass. Then, amorphous silicon is grown thereon. After the high temperature dehydrogenation process and the HF precleaning process, the laser of the ELA scans the amorphous silicon. The amorphous silicon melts at very high temperature and re-crystallized into poly-silicon.
The grain size of the low temperature poly-silicon possesses quite significant effect to the electricity of the poly-silicon. During the ELA process, the amorphous silicon suffered with the high temperature and nearly completely melts. Then, the re-crystallizion into poly-silicon of the amorphous silicon accomplished. As being re-crystallized, the occurrence of crystallization follows the direction from low energy toward high energy, i.e. crystallization occurs from the low temperature area toward the high temperature area; the start point and the direction of crystallization are in a mess. Consequently, the grain size is too small and too many grain boundaries appear. Accordingly, electron mobility of the poly-silicon can be significantly affected.